Different forms of the same gene are called alleles. A genetic locus is the spot on the chromosome where alleles for a specific gene can be found. Diploid organisms, including humans, have two alleles at each genetic locus because they inherit one allele from each parent.
Homozygous organisms contain two of the same allele, while heterozygous organisms contain two different alleles. The specific combination of alleles can affect an organism's phenotype, but in some cases there is no noticeable difference between individuals expressing two different alleles of the same gene.
In simple heredity, one allele is dominant and the other allele is recessive. In this case, a homozygous organism always displays the specific trait shared by both alleles, while a heterozygous organism displays the trait associated with the dominant allele. Round and wrinkled peas, which are respectively dominant and recessive, exhibit this type of inheritance pattern. Homozygous peas exhibit a round or wrinkled appearance depending on which allele they carry, while homozygous peas exhibit a round appearance despite the presence of one allele specifying for wrinkled skin.
More complex inheritance patterns also exist. Heterozygous organisms with incomplete dominance display a blended version of the two traits specified by the alleles. For example, a flower with an allele for red and an allele for white would be pink in a flower with incomplete dominance. Codominant organisms display both alleles separately. A codominant flower with red and white alleles may have some red petals and some white petals.